1. Field of the Invention
The present invention pertains to an electromagnetic clutch and particularly to a microclutch suitable for use in office automation or factory automation equipment.
2. Prior Art
FIG. 1 depicts a conventional electromagnetic microclutch of the aforementioned type. The electromagnetic clutch, generally designated by 1, comprises a hollow cylindrical rotor shaft 2 formed of a synthetic resin by integral molding. The rotor shaft 2 has an annular rotor 3 of a shallow channell-shaped cross-section securely mounted therearound. The rotor 3 has inner and outer circumferential magnetic poles 3a and 3b isolated from each other by through-grooves 3c. An annular yoke 4 of a deep channell-shaped cross-section is rotatably mounted on the rotor shaft 2 so as to be positioned on the left side in FIG. 1 with respect to the rotor 3. The yoke 4 holds a coil 6 therein and has a yoke-locking plate 7 securely fixed thereto. The yoke locking plate 7 is supported by equipment to which the clutch is mounted, and is kept stationary, so that the yoke 4 is prevented from accompanying the rotor shaft 2 when the latter is rotated. A gear 10 rotatably fits around the rotor shaft 2 and is prevented from axial movement by both the boss of the rotor shaft 2 and a retaining ring 11 mounted to the rotor shaft 2. A ring-shaped armature 8 is fixed to the gear 10 through an annular spring plate 9 so that one face thereof faces to the inner and outer magnetic poles 3a and 3b, leaving a small gap.
With such a construction, the armature 8 is rotated together with the gear 10 when a rotation force is transmitted from a motor (not shown) to the gear 10. During the rotation of the armature 8, the coil 6 is energized for magnetizing the yoke 4, so that a magnetic flux passes through the yoke 4, the inner circumferential magnetic pole 3a of the rotor 3, the armature 8, and the outer circumferential magnetic pole 3b, and then returns to the yoke 4, thus forming a magnetic path .PHI..sub.1. The armature 8 is hence brought into contact with the rotor 3 by magnetic attraction counter to the resilient force exerted by the spring plate 9, so that the rotor shaft 2 and hence an output shaft of the equipment are rotated.
The above-described electromagnetic clutch, however, has a number of problems, including:
(a) It has been difficult to accurately produce the yoke 4 by pressing since its cross-sectional configuration is of a deep channel shape.
(b) The two magnetic poles magnetically isolated from each other must be formed on the rotor 3. Therefore, the manufacture of the conventional electromagnetic clutch has been complicated and expensive.
(c) It has been very difficult to make the gap between the armature 8 and the inner circumferential magnetic pole 3a equal to that between the armature 8 and the outer circumferential magnetic pole 3b. Consequently, one of the gaps becomes larger, and the magnetic flux consumption tends to increase.
(d) Since the coil 6, the rotor 3 and the armature 8 are arranged in line in an axial direction, the resulting clutch is therefore relatively long in the axial direction. This results in unfavorable circumstances when mounting the clutch in equipment.